This invention relates to the control of power levels of transmitted signals in telecommunication systems, such as spread spectrum multiple access systems. In particular, it relates to control of transmit power levels of base stations involved in diversity communication with a mobile station.
Transmit power control is important to communication systems having many simultaneous transmitters to reduce the mutual interference of such transmitters. It is particularly important in communication systems that use code division multiple access (CDMA) to obtain high system capacity. Power control is important for the uplink, i.e., for transmissions from a mobile station to the radio network including several base stations, and for the downlink, i.e., for transmissions from one or more base stations to a mobile station.
Uplink power control may be provided by a closed-loop method where a base station measures the strength of a signal received from a mobile station and then periodically transmits one transmit power control (TPC) bit to the mobile station every certain time period, e.g., 1.25 milliseconds. Based on the received power control bit, the mobile station increases or decreases its transmit (uplink) power by a predetermined amount. On the downlink, one power control approach is for the mobile terminal to measure a received downlink power level from a base station and report the measurement back to the base station, which might adjust its transmit power in predetermined circumstances.
Downlink power control in a system compliant with the IS-95-A standard is based on frame error rate (FER) measurements by the mobile station, which sends FER reports to the system. Sections 6.6.4.1.1 and 7.6.4.1.1 of the IS-95-A standard note that such FER reports can be sent when a threshold has been crossed and/or periodically. Typically, an FER report may be sent every 1-5 seconds. One problem with this method is that it can take a long time (i.e., several seconds) to accumulate the appropriate FER statistics. As a result, Rayleigh fading and shadow fading are not tracked. In fact, the IS-95-A method has proved to be so slow that it is usually does not provide much added benefit compared to not using downlink power control.
Some personal communications systems (PCS) using CDMA are similar to the cellular IS-95 standard in that the mobile station reports downlink frame errors whenever they occur. This puts the radio network in control of frame errors, but it still takes a long time to accumulate the appropriate statistics. In other communication system concepts like the UMTS Code Division Testbed (CODIT), the signal quality is determined by estimating the raw bit error rate (BER) instead of the FER. Hence, good statistics can be obtained faster, and a mobile station sends BER reports to the network more often (i.e., 1-10 times per second). System performance is improved in comparison to a system using downlink transmit power control according to the IS-95-A standard, but the CODIT method is still too slow to handle Rayleigh fading.
The mobile terminal may also measure the downlink signal- to- interference ratio (SIR) of the signal received from the base station and transmit an appropriate power control command on the uplink back to the base station. Each power control command may be an uncoded, single bit in order to minimize signalling overhead. Assuming a smaller frame length than that of IS-95-A, a bit rate of several hundred kilobits per second, a wider channel bandwidth of 5 MHZ, and a CDMA chip rate of four million chips per second, such a downlink power control approach tracks Rayleigh fading fairly well.
The situation is more complicated if the mobile terminal is in a xe2x80x9csoftxe2x80x9d diversity handover mode. Soft handover is also called macrodiversity and describes a situation where the mobile terminal communicates with two or more base stations simultaneously. Soft handover is described in U.S. Pat. No. 5,109,528 to Uddenfeldt and U.S. Pat. No. 5,327,577 to Uddenfeldt, both of which are incorporated here by reference.
When the mobile terminal is not in soft handover mode, the error rate of the uncoded power control commands should be small, e.g., about one percent, in normal transmission conditions. But the error rate of the uncoded downlink power control commands becomes a more serious problem when the mobile terminal is in soft handover. Because there are different paths or links between the mobile station and each of the handover base stations, and different conditions affect each of those paths, the errors impacting those commands vary across the radio links. These errors are typically independent so that different errors will likely differently impact the same command sent to each base station involved in the soft handover. A large number, e.g., 1500, of power control commands per second may be sent to two base stations involved in a soft handover in order to track rapid channels changes, e.g., Rayleigh fading. The result is that the average transmit powers of each of the base stations involved in the diversity handover may xe2x80x9cdriftxe2x80x9d apart from where they should be. As a result, the commanded transmit power levels of the base stations drift apart to suboptimal levels from a system-capacity point of view.
The lost capacity occurs because at least one of the base stations in communication with a mobile station in soft handover mode will transmit at a power level that is too high. The magnitude of the difference between one downlink power level and another downlink power level affects the system""s capacity because one base station""s transmissions look like interference to other base stations. In addition, the rate that each downlink power level drifts is important because faster rates usually require the control commands to be issued more frequently. Higher frequency control commands generally increase the messaging load that must be carried by the control links between the base stations BS1, BS2 and the RNC.
Several ways to combat base station power drift include reducing the probability of transmit power control command errors by sending fewer such commands, increasing bit redundancy of the transmit power commands, or reducing the size of the power adjustment made at the bast station in response to each command. Unfortunately, all of these approaches are not completely satisfactory because rapid changes in the radio channels are not tracked.
Alternatively, the power drift may be reduced by increasing control signaling between the base stations via one or more radio network control nodes to synchronize the downlink transmit powers of those base stations. Namely, the power transmission level of each base station in the diversity handover is compared to a power reference or threshold common to all the base stations in the diversity handover. The difference between the transmit power of each base station and the reference power is used to correct the transmit power level of that base station. Because the power correction depends on the difference between the actual transmit power at the base station and the common reference power, the various transmit powers of the different base stations in the diversity handover converge relatively quickly. Thus, even if the transmit power command from the mobile station is received in error in one or more of the base stations, the power correction based on the comparison to the common reference power compensates for such errors.
A disadvantage with this alternative is the increase in control signaling. This becomes even more of a problem the more frequently the reference power is updated. However, more frequent updates are desirable to track rapid channel changes.
Another problem with this alternative is that updating a single reference power for all the soft handover base stations does not permit the flexibility to set different diversity handover power references for different base stations. For example, it may be desirable to set the reference power of a dominant or favored base station to a higher value than the other base stations involved in the diversity handover.
The above problems are overcome by the present invention. It is an object of the present invention to provide both effective and flexible power control of the base stations involved in a macrodiversity transmission to a particular mobile station to minimize base station drift. It is a further object of the present invention to reduce signalling requirements between base stations involved in a macrodiversity transmission and a radio network control node when adjusting the transmit power levels of the base stations.
In the present invention, the downlink transmit powers of base stations involved in a diversity handover are controlled to minimize base station power xe2x80x9cdrift.xe2x80x9d A single reference power level is determined based on detected transmit power levels of the base stations. A power offset is determined for one or more of the base stations based on signal quality measurements made by the mobile station. The reference power level and the power offset are used to regulate an adjustment to the respective transmit power levels of the base stations for the diversity handover communication. The reference power is embedded in the user data stream and transmitted at a relatively high frequency to all of the base stations in the diversity handover. The high frequency transmission of adjusted reference power from the radio network control node to the diversity handover base stations tracks rapid channel fluctuations and compensates for independent errors in frequently transmit power control (TPC) commands from the mobile station. In other words, the high frequency reference power adjustment minimizes the xe2x80x9cdriftxe2x80x9d between base station transmit powers. By embedding the adjusted reference power in the user data stream, the associated signalling load is kept to a minimum.
Flexibility in reference power setting is achieved using a companion procedure. Individual power xe2x80x9coffsetsxe2x80x9d from the frequently broadcast power reference are sent to associated individual base stations using selective signalling over a control channel. Control channel signalling, although more xe2x80x9ccostlyxe2x80x9d in terms of signalling overhead, allows a single base station to be directed with individual commands. Added signal processing overhead is kept to a minimum because the offsets are transmitted from the radio network control node at a lower frequency than the higher frequency at which the reference power is adjusted. The control signaling permits individual addressing of the control frame to a specific base station or radio link rather than to all of the base stations or radio links involved in the diversity handover.
The lower frequency, companion procedure provides the flexibility of setting different transmit power levels for different base stations involved in the diversity handover using different offsets from the general reference power level established by the fast base station power synchronization procedure. This flexibility may be desirable in situations where one of the base stations is a favored or dominant base station in the communication. For example, the favored or dominant base station may transmit at a higher power level, while less favored/dominant base stations are set to transmit at lower power levels to reduce unnecessary interference, but still obtain the benefits associated with diversity handover.